The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a stop of the vehicle.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. Output of an engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
In a control system described in EP-A 0182616, a drive pulley speed (engine speed) sensor, a driven pulley speed sensor, and a throttle position sensor are provided. Actual transmission ratio (i) is calculated by a calculator based on the output signal (N.sub.P) of the drive pulley speed sensor and the output signal (N.sub.S) of the driven pulley speed sensor with a formula (i=N.sub.P /N.sub.S). Desired transmission ratio (id) is calculated based on the output signal (N.sub.S) of the driven pulley speed sensor and the output signal (.theta.) of the throttle position sensor. The actual transmission ratio i is controlled to the desired transmission ratio id by shifting the spool of the transmission ratio control valve.
In such a system, when the vehicle stops, the drive and driven pulleys stop, and hence the transmission ratio at that time can not be calculated based on the speeds of both pulleys. In an ordinary decelerating operation, the transmission ratio gradually increases, and the ratio reaches a maximum ratio when the vehicle stops.
However, when the vehicle is rapidly decelerated, the vehicle is stopped before the transmission ratio reaches the maximum ratio, and the belt stays in a lower ratio position than the maximum transmission ratio position. In such a state, when an accelerator pedal is depressed in order to re-start the vehicle, the belt may slip on the pulleys because of the lower transmission ratio. The slipping causes wear and tear of the belt.